The invention relates to an electrical resistor formed in an integrated semiconductor circuit.
Electrical resistors find application, in particular in a resistor chain as used, for example, for attenuation or amplification of audio signals.
Integrated resistors are formed by resistance regions of semiconductor material which extend between spaced apart terminal contacts. A resistance region may be a diffusion region of a first conductivity type diffused into the surface of a diffused or implanted pocket of the opposite conductivity type. An example hereof is shown in DE 35 26 641 A1. The resistance region may also be a diffusion region of a first conductivity type diffused into the surface of an epitaxial pocket of the opposite conductivity type. Examples thereof are shown in EP 0 001 574 A1 and EP 0 017 919 A1. Moreover, the resistance region can be formed by an epitaxial pocket of a first conductivity type that is formed on a substrate of the opposite conductivity type. In all cases there is a pn junction between the resistance region and the semiconductor material located therebeneath. The semiconductor material located underneath the resistance region has such a potential applied thereto that it is ensured between resistance region and semiconductor material therebeneath, in case of all voltages applied to the terminal contacts of the resistance region, that the pn junction between resistance region and semiconductor material therebeneath is biased in blocking direction, thereby forming, for isolation, a charge-carrier depleted space-charge region between resistance region and semiconductor material located therebeneath. With changing difference between the potential arising in the resistance region and the potential arising in the semiconductor material located therebeneath, the breadth (extension) of the space-charge region changes, which has an effect on the resistance value of the integrated resistor and on the voltage drop along the resistance region of the integrated resistor.
EP 001 574 is not concerned with this problem. This document discloses a semiconductor structure which is flexible with respect to its final usability and in which an Nxe2x88x92 epitaxial layer located on a substrate first has a P+ region formed therein, with an Nxe2x88x92 region being formed in the surface of the latter. In accordance with the type of contacting, the three-layer structure obtained in the epitaxial layer can be utilized for realizing a transistor or for realizing two resistors on top of each other which are separated from each other by the central layer. Resistance changes arising with changing potential due to the correspondingly changing space-charge region are not avoided in this known structure.
DE 35 26 461 A1 is not concerned with the problem mentioned either. In case of this document, the dependency of the linearity of the divisor ratio of an integrated resistor chain on the contact resistances of the contacting regions of the individual resistors of the resistor chain is counteracted by forming the individual resistors not in one single common resistance path, but by forming each individual resistor in a separate resistance path with two terminal contact regions.
In case of the semiconductor structure known from EP 0 017 919 A1, the dependency of an integrated resistor on changes in the space-charge region isolating the resistor and dependent on potential changes, is reduced by forming the resistor by two series-connected resistance regions each having associated therewith a respective insulating region of its own of an epitaxial layer located underneath the resistance regions, which both have different potentials applied thereto. In said document, such potential application to the individual resistance regions and the individual insulating regions is carried out that, in case of a change in voltage applied to the two resistance regions, a resistance reduction of the one resistance region and a resistance increase in the other resistance region compensate each other.
When the integrated resistor is utilized for attenuation of an alternating voltage signal, for example an audio signal, the potential difference between the resistance region and the semiconductor material therebeneath changes in accordance with the alternating potential applied to the resistance region. This effects a corresponding modulation of the space-charge region formed at the pn junction. This in turn effects modulation of the resistance value of the integrated resistor and of the voltage drop along the resistance region. When the integrated resistor is used for attenuation of an audio signal, this causes distortion with a specific distortion factor.
Reduction of this distortion factor was obtained by integrated resistors, the resistance region of which is formed by a layer of polycrystalline silicon located on an insulating layer, preferably an oxide layer, which mostly is formed on the surface of an epitaxial pocket or directly on the substrate. With variable potential difference between this resistance region and the semiconductor material located therebeneath, impairment of the integrated resistor occurs in case of such an integrated resistor as well, which in particular consists in resistance modulation in case of alternating voltages and in distortion in case of audio signals. This is due to the fact that, as a result of effects in accordance with the field strength on the bottom side of the polycrystalline resistance region directed towards the oxide layer, charge carrier concentrations result that change with the potential difference between the resistance layer and the semiconductor material located beneath the oxide. This mechanism is comparable to the effects of the gate potential of a MOS transistor on the channel region located underneath the gate oxide.
In a practical version of a conventional integrated audio circuit in which such resistors of polycrystalline silicon are used with a voltage dependency of approx. 100 ppm, audio voltage dividers with 6 db attenuation at 1 Vrms at the input of the divider obtained a distortion factor of 0.004 percent at the output of the divider. Still better results are obtained when resistors are used whose resistance region consists of an oxide-insulated silicon-chromium layer. However, the manufacture of such silicon-chromium resistors is very complex and expensive.
Although integrated resistors with resistance regions in the form of polycrystalline silicon layers or silicon-chromium layers result in lower distortion factors than integrated resistors in the form of diffused resistance regions or in the form of epitaxial resistance regions, there are applications for which a reduction in distortion factor is desired to a value that is still markedly lower than that distortion factor that can be obtained using polycrystalline and silicon-chromium resistance regions, respectively.
According to disclosed embodiments of the invention, the foregoing can be achieved by forming in the semiconductor material underneath the useful resistor region an additional auxiliary resistor region such that an equivalent voltage drop curve arises along the useful resistor region and along the auxiliary resistor region.
An integrated resistor according to disclosed embodiments of the invention comprises a useful resistor having two spaced-apart useful resistor terminal contact regions for connection of useful resistor contacts and a useful resistor region of semiconductor material located therebetween. An auxiliary resistor having two spaced-apart auxiliary resistor terminal contact regions for connection of auxiliary resistor terminal contacts and an auxiliary resistor region located therebetween is provided in the semiconductor material underneath the useful resistor region. The useful resistor region and the auxiliary resistor region are separated from each other by an electrically insulating intermediate region. The useful resistor region and the auxiliary resistor region have substantially identical topographies. The auxiliary resistor terminal contact regions are each connected to the same electrical potential as the respectively adjacent useful resistor terminal contact regions, or the potential thereof is each shifted by the same d.c. voltage potential difference for improved isolation.
Without application of a potential shift and with exactly identical topography of useful resistor region and auxiliary resistor region, there is created, at all locations along the useful resistor region, a potential difference of zero between useful resistor region and auxiliary resistor region. Upon application of an a.c. voltage both to the useful resistor and to the auxiliary resistor, there are indeed arising changes in time of the potential values in the useful resistor region and the auxiliary resistor region corresponding to the a.c. voltage curve. The potential difference between useful resistor region and auxiliary resistor region, however, remains about the same at all times and at all locations along the useful resistor region. Resistance changes and resistance modulations thus hardly occur in the useful resistor. Resistance changes in the auxiliary resistor, however, do occur because of the changing potential difference between auxiliary resistor and substrate located therebeneath. The change in resistance due to voltage dependency is thus quasi shifted from the useful resistor to the auxiliary resistor where it has no disadvantageous effects.
In case of integrated resistors in which the useful resistor region is separated from the auxiliary resistor region by a pn junction, it is sufficient for electrical isolation between useful resistor region and auxiliary resistor region if a blocking biasing voltage is present across the pn junction, i.e., a biasing voltage lower than the threshold voltage at which the pn junction reaches the conducting state. The potential difference across the pn junction thus may be zero, which is the case if the topographies of useful resistor region and auxiliary resistor region are exactly identical and if the auxiliary resistor terminal contacts and the respectively adjacent useful resistor terminal contacts have the same potential applied thereto. However, it must be avoided in all cases that a voltage difference occurs at the pn junction that brings the pn junction to the conducting state. It is true that the potential difference of zero is sufficient for isolation, but care is to be taken that the highest voltage dependency in case of diffused resistors is present especially in the range of low blocking voltages and that minor errors in the voltage drop curve of the auxiliary resistor may still have a noticeable effect.
With practical applications, it will be difficult to obtain the ideal case that the useful resistor region and the auxiliary resistor region have exactly identical topographies. This leads to certain deviations in the voltage drop curves of the useful resistor region on the one hand and the auxiliary resistor region on the other hand. In particular when larger deviations between the topographies of useful resistor region and auxiliary resistor region should be or must be tolerated for manufacturing reasons, along with the concomitant deviations of the voltage drop curves of useful resistor region and auxiliary resistor region, it is possible to provide between the useful resistor terminal contact regions and the respectively adjacent auxiliary resistor terminal contact regions one d.c. voltage source with identical voltage each, instead of connecting the auxiliary resistor terminal contact regions to the same potential as the useful resistor terminal contact regions. By means of this d.c. voltage source, the pn junction between useful resistor region and auxiliary resistor region is biased such that it is ensured also in case of deviations of the voltage drop curves of useful resistor region and auxiliary resistor region that the pn junction located therebetween definitely does not reach the conducting state and the blocking voltage in total becomes higher and thus is within the range of low voltage dependency.
In audio circuits, the application of the measures according to the invention permits, with the manufacturing technology remaining the same, a considerable reduction in distortion factor to be achieved, or it is possible to change to a simpler manufacturing technology which, without the measure according to the invention, would result in a deteriorated distortion factor. For example, if a conventional integrated resistor were used as basis, having its resistance region formed by a polycrystalline silicon layer, it is possible either to obtain a reduction in the distortion factor, if this is demanded, by addition of an auxiliary resistor to this polycrystalline resistor, or it is possible, if the distortion factor obtained with the conventional polycrystalline resistor is sufficient, to change, for example, to a diffused resistor and associate an auxiliary resistor with the same. If the distortion factor need not be improved, such change of technology is advantageous since it is possible to use for diffused resistors manufacturing technologies requiring no additional masks, as they are necessary for producing polycrystalline resistors. When no improved distortion factor with respect to the conventional polycrystalline resistor is required, lower manufacturing costs may be achieved by changing over to a diffused useful resistor along with an additional auxiliary resistor.
Associating an integrated auxiliary resistor with an integrated useful resistor is advantageous in case of single resistors, but in particular in case of resistor chains (for resistance dividers/voltage dividers). The disclosed embodiment of the invention is especially advantageous for resistors having a.c. voltage signals, for example audio signals, applied thereto since the resistance modulation is fully or to the largest possible extent eliminated by the a.c. voltage signal. However, the invention is also advantageous for resistors having d.c. voltages only applied thereto, in particular in circuits requiring high-precision resistance conditions. With conventional precision resistance dividers, it was either necessary to ensure a high-precision resistor topography and to consider the resistance values changed by the voltage dependency, or to trim the resistors emanating from the manufacturing process by means of laser beams. Both methods result in correspondingly high manufacturing costs.
Due to the fact that an integrated resistor made in accordance with the invention results in exact, voltage-independent resistance conditions also with simple manufacturing methods, such cost-increasing measures can be avoided.